Recently, it becomes prevailing rapidly to use the Internet while a user moves. As a result, number of mobile terminals, which can deal with high speed packet communication, out of the cellular phone, the smartphone or the like increases. As a standard to realize the high speed packet communication, W-CDMA (Wideband Code Division Multiple Access), HSPA (High Speed Packet Access), LTE (Long Term Evolution) and the like have been proposed, and each mobile terminal uses the above-mentioned standard. However, by using the high speed packet communication, electric power consumption of the mobile terminal becomes increasing. As a result, in the case of a mobile terminal which works with a battery, a user's available time at a time when the user goes out becomes decreasing.
Then, according to PTL 1 (Patent Literature 1), a RRC (Radio Resource Control) state is set in a radio device of the mobile terminal. By setting the RRC state, the mobile terminal transits to a state in which high speed communication can be carried out only when packets are received frequently, and consequently it is possible for the mobile terminal to receive data instantaneously while electric power consumption is large instantaneously. Furthermore, when the mobile terminal does not receive any packets for a predetermined period of time, the mobile terminal transits to a suspension state from a low speed communication state by setting the RRC state. As a result, it is possible to suppress the electric power consumption. PTL 1 discloses the above-mentioned method for controlling the RRC state.
According to NPL (Non Patent Literature) 1 and NPL 2, in order to express the RRC state which is set at a time when receiving the packet, CELL_DCH (Dedicated Carrier Channel) is assigned to a high speed communicable state, and CELL_FACH (Forward Access Channel) is assigned to a low speed communicable state, and CELL_PCH (Paging Channel) and UTRA_Idle (Idle) are assigned to a suspension state, in the case of W-CDMA (HSPA). Moreover, in the case of LTE, RRC CONNECTED (connected state, CONN) is assigned to a communicable state, and RRC IDLE (idle, RRC idle) is assigned to a suspension state. Battery's electric power consumption of DCH, FACH, PCH and Idle becomes small in this order in the case of W-CDMA (HSPA), and battery's electric power consumption of CONN and RRC IDLE becomes small in this order in the case of LTE. Here, a radio transmission method is selected out of LTE and W-CDMA (HSPA) according to a radio network with which the mobile terminal is connected.
Moreover, in order that the mobile terminal, which is in the suspension state, may resume receiving data, it is necessary that the mobile terminal transits to the high speed communicable state or the low speed communicable state. In order that the RRC state may transit, it is necessary to carry out a negotiation between a network apparatus and the mobile terminal which are included in a radio network. A delay time in the RRC state transition amounts to several hundred milliseconds in the case of LTE, and about 2 to 5 seconds in the case of W-CDMA. As mentioned above, the mobile terminal in the communicable state transits to the low speed communication state or the suspension state if the mobile terminal does not receive any packets for the predetermined period of time. The predetermined period of time (inactivity timer) is specified in the radio network or the mobile terminal in advance.
Moreover, push type communication, which is used in order that a service provider on the Internet may distribute any message at any point of time to the mobile terminal, has started. In WWW (World Wide Web) which is used in the Internet, pull type communication, in which the mobile terminal starts communication with a WWW server on the Internet, is used. The push type communication is different from the pull type communication in a point that a service server, which the service provider holds, starts communication with the mobile terminal. While the push type communication is independently implemented in each application which provides a service, a general-purpose base for providing the push service is being recently built from a view point of efficient utilization of the network.
According to the base for the push service, a push server or a server which is called the push proxy gateway is arranged in a network of a push provider. Then, the push server or the push proxy gateway server receives a push request including a message (push message), which the service provider wants to push, from the service provider. Then, the push server identifies a mobile terminal, which is included in the push request, as a destination terminal, and distributes the push message to the mobile terminal by packet communication through the radio network. When carrying out the packet communication with the mobile terminal at a time of distribution, it is necessary to make the RRC state of the mobile terminal enter into the communicable state.
As a method of communication between the mobile terminal and the push server, TCP (Transmission Control Protocol)/IP (Internet Protocol) protocol is used in general. FIG. 1 shows a connection activating sequence according to the TCP/IP protocol. According to the TCP/IP protocol, a connection source apparatus, which activates the connection, transmits the SYN packet to a connection destination apparatus, and the connection destination apparatus replies the SYN+ACK packet to the connection source apparatus, and the connection source apparatus replies the ACK packet to the connection destination apparatus. As a result, the 3 way handshake for establishing the connection is carried out.
As a method which uses the TCP/IP protocol between the mobile terminal and the push server, two methods can be conceived. One is a method (keep alive method) in which the mobile terminal activates the TCP/IP connection with the push server, and the connection is maintained. The other is a method (direct connection method) in which the push server activates the TCP/IP connection with the mobile terminal every time when distributing the push message.
According to the keep alive method, the push server can communicate with the mobile terminal even in the case that NAT (Network Address Translation) or a proxy exists between the mobile terminal and the push server. On the other hand, according to the direct connection method, the push server can not recognize an IP address which identifies the mobile terminal, and consequently cannot communicate with the mobile terminal. However, in the case of the keep alive method, it is necessary to always maintain the TCP/IP connection between the mobile terminal and the push server, and consequently a load to be processed becomes heavy. On the other hand, in the case of the direct connection method, the TCP/IP connection is carried out only for a short period of time which is necessary for carrying out the push distribution. As a result, the load to be processed becomes light.
According to the TCP/IP protocol, when a packet transmission source transmits a packet, a timer which is called RTO (Retransmission Time Out) is worked. In the case that the ACK packet, which indicates that a packet transmission destination normally receives the packet, is not replied by the packet transmission destination within a period of time designated by the timer, the packet transmission source judges that the packet is lost during transferring, and then the packet transmission source retransmits the same packet. RTO has a value which is defined each time when the TCP/IP connection process is carried out. In NPL 3, an initial value of RTO is calculated according to the following Formula 1.RTO=1 second(=3 seconds if arrival of SYN+ACK packet is delayed)  (Formula 1)When firstly receiving the ACK packet in the TCP/IP connection process, RTO is calculated according to the following Formula 2.RTO=RTT+K×RTT/2  (Formula 2)Here, RTT is a round trip time which is defined as a time from the packet transmission source's transmitting the packet until the packet transmission source's receiving the ACK packet corresponding to the packet. Moreover, each time when the round trip time is calculated afterward, RTO is calculated according to the following Formula 3.RTO=SRTT+K×RTTVAR(K=4)  (Formula 3)where SRTT is an average of the round trip times of the past packets, and RTTVAR is an average variation of the round trip time. In the case that the ACK packet corresponding to the transmitted packet is not replied during RTO, the transmitted packet is retransmitted, and the back-off algorithm makes the value of RTO two times as long as the current value, and then the packet transmission source waits for the ACK packet's arriving.
In the case of a network whose delay time varies severely, the round trip time, which is measured at a time when the delay time is short, becomes short. As a result, the value of RTO becomes small. Afterward, when the transmission delay time changes rapidly so as to have a large value, RTO which has the small value causes frequent retransmission of the packet. According to PTL 2, in the case that a transmission terminal transmits a packet with frequency which is higher than predetermined frequency, a receiving side terminal does not reply the ACK packet instantaneously and replies the ACK packet after an elapse of short time. As a result, RTT becomes long. That is, by making RTO of the transmission side long always, it is possible to make number of re-transfers of the packet, which is caused by an expiration of RTO, small.